A glass tube used to study the electrical conduction through gases at low pressure is known as a discharge tube.
(I)At a pressure of about 10mm of Hg
When the pressure in the discharge tube is reduced to about 10mm of Hg. The blue streamers are formed with the cracking raise.
(II)At a pressure of 5mm of Hg.
When the pressure in the discharge tube is reduced to about 5mm of Hg, then the luminous columns called Geiosler’s discharge are formed. The colour of the discharge depends are formed colour of the discharge depends on the nature of the gas.
(III)At a pressure of 2mm of Hg
When the pressure in discharge in the discharge tube is reduced to about 2mm of Hg a long Luminous Column appears from anode to cathode called a positive column. Colour depends upon the nature of the gas.
(IV)At a pressure of 1mm of Hg.
When the pressure in the discharge tube is reduced to 1mm of Hg, the positive column detached itself from the cathode and moves the anode. A Luminous glow appears at the cathode called a negative glow. The dark space between positive column and negative glow called faraday’s dark space.
(V)At a pressure of 0.5mm of Hg.
When the pressure in the discharge tube is reduced by about 0.5mm of Hg. The positive column gets shortened, the Faraday dark space extended to a greater length. The negative glow leaves the cathode and another glow appears on the cathode called cathode glow. The negative glow moves towards the anode. The space between cathode glow and a negative glow appears dark and is called Crooke’s dark space.
(VI)At a pressure of 0.05 of Hg.
In this range of pressure, the positive column shortens and breaks into an alternative bright and dark disc called striation.
(VII)At a pressure of 0.01 mm of Hg.
At this pressure, first striations disappear and negative and cathode glow vanishes. The whole tube is filled with crooks dark space At this stage, the luminous ray is filled with crooks dark space At this stage, a luminous ray is seen to come out of the Cathode. These rays are called the cathode rays.
Cathode rays are the electrons travelling from the cathode to the anode. The walls of the tube are seen to glow with blue or green light, depending on the nature of the glans.
The invisible rays, emerging normally from the cathode of a discharge tube, kept at a pressure of 0.01mm of Hg and under a very high potential difference of the order of 10-15 KU is called Cathode Rays.
These are the conceptual notes of gaseous discharge at various pressure
Properties of cathode rays
(I)Cathode rays are emitted normally from the surface of the cathode.
(II)Cathode rays can penetrate the small thickness of matter such as sheets of aluminium foil.
(III)Cathode rays carry a negative charge. So they are deflected by an electric and magnetic field.
(IV)Cathode rays carry momentum and kinetic energy.
(V)They travel with 1/2 the velocity of light.